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Release It! by Michael T. Nygard

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Gathering Availability Requirements

Asking the sponsors of your system, “How highly available should this be?” probably results in one of two answers. The less experienced sponsors will simply reply, “100%.” The more knowledgeable will say, “five nines” because it sounds cool and technical. But are five nines really required?

The proper way to frame the availability decision is in straightforward financial terms: actual cost vs. avoided losses. For example, “98% availability” translates to 864 minutes of downtime each month. That downtime has a direct cost because of lost revenue. Suppose the site brings in $1,500 per hour during the peak of the day—the worst possible time to be down. Then the worst-case loss with 98% availability is about ...

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